1. Field of the Invention
The present invention relates to a micro fuel cell system applicable to an electronic device, and more particularly, to a micro fuel cell configured by processing a flexible polymer material.
2. Description of the Related Art
There have been active developments on small-sized fuel cells used for notebook computers, wireless pagers, digital camcorders, cameras, hearing aids and portable power sources, and micro fuel cells used as power sources for the next-generation MicroElectroMechanical Systems (MEMS). These fuel cells involve mechanically processing conventional materials (silicon, Polydimethylsiloxane (PDMS), etc.) by forming micro holes and channels using a semiconductor integrated circuit process and the conventional MEMS technology.
In order to manufacture a conventional micro fuel cell, researches have been conducted on application of a silicon substrate to the micro fuel cell. Based on the research results, the fuel cell shown in FIG. 1 has been experimentally manufactured. The system is characterized by connecting several pieces of unit cells of planar electrolyte membranes and electrodes in series and disposing the unit cells between thin disc-shaped Si substrates each having a diameter of about 10 cm. In the Si substrates, micro gas channels were made using micromachining techniques. However, the micro fuel cell with such silicon substrates is manufactured by the semiconductor integrated circuit process, resulting in high manufacturing costs. Moreover, it is not easily applicable as a power supply source for a mobile electronic device such as a mobile phone and a Personal Digital Assistant (PDA) due to fragility of silicon. In addition, the interfacial bonding between the silicon and Nafion®, commercially available from Dupont, USA, is not good, thus negatively affecting the efficiency of the fuel cell.
In the case of a silicon fuel cell applicable to a mobile electronic device adopting the integrated circuit process and using silicon material, micro channels for supplying fuel are formed in the silicon by etching, and metallic material is deposited on the surface of the electrode by sputtering, etc. to function as a current collecting layer of electrons. This silicon fuel cell, however, has weak interfacial bonding with Nafion®, which makes up the membrane of the fuel cell system, and thus may have cracks formed in Nafion® due to the differences in Coefficients of Thermal Expansion (CTE) or hygroscopicity, resulting in a fatal defect of the system. In addition, due to the fragility of silicon, the fuel cell is not applicable to a mobile electronic device as a power source.
In this regard, “Proceedings of the 14th IEEE International Conference on Micro Electro Mechanical Systems”, pages 21-25 by W. Y. Sim, G. Y. Kim and S. S. Yang (refer to FIG. 2), and “Electrochimica Acta” 49(2004), pages 821-828, by G. Q. Lu, C. Y. Wang, T. J. Yen and X. Zhang (refer to FIG. 3) disclose technologies aimed to optimize an electrode structure to enhance the performance of the electrodes and precisely design a stack structure to minimize the internal volume and increase the output density per volume using the integrated circuit process. The integrated circuit technology facilitates miniaturization using the extensive technologies accumulated to date. The silicon fuel cell system using silicon as the material of the cell may be manufactured in the form of a thin film. The flow channels are formed in the silicon substrate using an etching process of semiconductor, with Membrane Electrode Assembly (MEA) formed over the micro channels via deposition, etc. As indicated in the non-patent documents 1 and 2, researchers have suggested different methods of forming tens of micro-units of holes or channels in the silicon substrate.
Such silicon material is advantageous for micro-sized machining, but it incurs high manufacturing costs since it requires the semiconductor integrated circuit process. In addition, formation of channels in the fuel cell with silicon material requires a lithography process or mechanical and chemical etching processes, resulting in a complicated manufacturing process.
Korea Patent No. 0494307 discloses a conventional technology in which a photosensitive polymer material is coated on a glass substrate by spin coating and exposed to ultraviolet rays to form a pattern, and eliminated from the glass substrate via a lift-off process, thereby obtaining a photosensitive polymer structure (refer to FIG. 4). In this fuel cell, the polymer material is used to form the structure, thereby reducing contact resistance to improve performance. In addition, the fuel cell is manufactured in a simple process with its thickness and design easily modifiable, which in turn facilitates mass-production.
However, the photosensitive polymer material has a drawback that it breaks up easily with application of strength exceeding a certain degree although it is improved in mechanical strength after undergoing exposure to light and baking. In addition, the photosensitive polymer material remaining in the final product without being stripped after lithography is too expensive, thus not applicable in practice.